Error messages generated in 'Customize Your Site'

I’ve recently installed WordPress locally on my Mac. Unfortunately though when I click on Customize Your Site, the loading circle just keeps spinning and spinning. This is the case in Safari, Firefox and Chrome.

To resolve the problem, so far I have

1. Reinstalled WordPress, with fresh MySQL database and all;
2. Checked that the problem persists in all browsers;
3. Flushed all caches and cookies;
4. Enabled SCRIPT_DEBUG (should I disable this now, by the way?);
5. Performed diagnostics using the error consoles on all three browsers.

Below are the error codes that were generated on each browser:

Safari: [Error] Failed to load resource: the server responded with a status of 412 (Precondition Failed) (localhost, line 0)

Firefox: downloadable font: rejected by sanitizer (font-family: "dashicons" style:normal weight:normal stretch:normal src index:0) source: https://localhost/wp-includes/fonts/dashicons.eot

Chrome: GET https://fonts.googleapis.com/css?family=Open+Sans%3A300italic%2C400italic%2C600italic%2C300%2C400%2C600&subset=latin%2Clatin-ext&ver=4.3.1 net::ERR_PROXY_CONNECTION_FAILED

I can provide screenshots of these error messages if necessary.

Could my problem have something to do with the fact that I’ve installed it locally?

Any advice is much appreciated! ??